Solve for $z$ : $15 + z = 24$
Explanation: Subtract $15$ from both sides: $ \begin{eqnarray} \\ 15 + z &=& 24 \\ \\ {-15} && {-15} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{15 + z} &\hphantom{=}& \hphantom{24} \\ z &=& 24 {- 15} \\ \end{eqnarray} $ Simplify. $ z = 9$